1. Field of the Invention
The present invention relates to the field of current amplifiers having an output stage comprised of MOS transistors. The present invention more specifically applies to a frequency compensation of an output stage comprised of a MOS power transistor.
2. Discussion of the Related Art
FIG. 1 shows a conventional example of a current output amplifier having an output transistor in MOS technology.
An output stage of the amplifier is comprised of a MOS transistor M1 (in this example, a P-channel transistor) receiving a control current Ic and mounted in a current mirror with a MOS transistor M2 of the same type. The sources of transistors M1 and M2 are connected to a positive supply line V+. The drain of transistor M2 constitutes an output terminal S of the amplifier. Terminal S is meant to receive a first terminal of a load Z, for example having an inductive component, a second terminal of which is connected to a negative supply line V- (for example, the ground). The gate of transistor M1 is connected to its own drain and to the gate of transistor M2. The drain of transistor M1 constitutes a terminal E for controlling the amplifier with a current. The surface area ratio of transistors M1 and M2 determines the amplification ratio between the current Ic imposed in the drain of transistor M1 and the output current Is supplied to load Z. The current control has been symbolized by a variable current source 1 between terminal E and line V-. The amplifier assembly shown in FIG. 1 constitutes a generator of variable current Is.
A disadvantage of such an assembly is that it does not operate satisfactorily in the high frequency range (current Is higher than 500 mA) when output current Is is desired to rapidly follow a variation of control current Ic for a high amplification ratio.
For the transistor response to be fast, its equivalent parasitic capacitance brought back on the gate has to be low. This so-called gate capacitance corresponds to the parallel gate-drain and gate-source capacitances. Although the gate-source capacitance generally is higher than the gate-drain capacitance, the latter is inversely proportional to the drain-source voltage. In the high frequency range, it thus has a much higher potential excursion than the gate-source capacitance and its influence over the frequency response of the amplifier thus cannot be neglected. When current Is in load Z increases, the drain-source voltage decreases and the gate capacitance increases with the increase of the gate-drain capacitance. The gate capacitance can reach values of approximately 10 to 20 nanofarads for a current Is of approximately 1.5 amperes.
Such a parasitic capacitance strongly alters the frequency response of the amplifier. Indeed, the output stage introduces a cut-off frequency which is, as a first approximation, proportional to the ratio of the gain transconductance (ratio of the drain current over the gate-source voltage) of transistor M1 over the gate capacitance of transistor M2. The cut-off frequency which varies according to the output current is low for significant currents (approximately one kHz for a current of approximately 1.5 amperes). As a result, the amplifier so constituted has a very low gain for frequencies exceeding approximately 1 kHz.